How to add or edit a custom class on a Pod
Login to the CMS.
In the tree menu on the left-hand side, go to any page (or add a new page).
At the label 'Main content', click on an existing Pods component (or add a new Pods component).
In the slide-out panel, click on an existing Pod (or add a new Pod).
In the top-right, go to the Pod Settings.
At the label 'Custom classes', enter your custom class (or classes — be sure to leave a space between each class).
Click 'Submit' in the bottom right, until you return to the main page node.
Hit 'Save and publish'.